A typical two-cycle internal combustion engine includes a cast metal engine block defining a plurality of cylinders. With a cross flow or direct charge system, a transfer passage is associated with each cylinder and extends from the crankcase end of the block to the upper end of the respective cylinder and a pair of transfer ports provide communication between each transfer passage and the cylinder. During operation, the fuel/air charge is drawn through the transfer passage and enters the cylinder through the two intake ports. The split charge converges and is directed upwardly by a curved deflector on the piston. The converging charges travel in a course to the top of the combustion chamber and then down the sloping side of the deflector to remove the spent gases from the previous power stroke through exhaust ports.
The conventional two-cycle engine block utilizing a cross flow charge system is cast with a series of openings, each of which extends through the wall of the block and communicates with a cylinder, and a separately cast metal port cover is connected to the block and encloses the openings. The inner surface of the port cover defines the outer walls of the transfer passages.
To assemble the port cover with the cast block, the mating surfaces on the cover and block must be machined and holes are drilled in the cover and block and tapped. The mating surfaces are then gasketed and secured together by bolts. The assembly procedure involves a substantial labor and material cost. Further, in order to provide adequate gasketing, the mating surfaces of the port cover and the block must have a substantial width or section, and this increases the overall weight of the block.
Evaporable foam patterns made of polymeric material, such as polystyrene, have been used in the past to cast metal components of internal combustion engines. In the evaporable foam casting process, a pattern is fabricated from the polymeric material and has a configuration conforming to the metal part to be cast. The pattern is then placed in a mold and the area around the pattern, as well as the cavities in the pattern, are filled with an unbonded flowable material, such as sand. Molten metal is introduced to the pattern via a sprue and the heat of the molten metal will vaporize the foam material, with the vapor being entrapped within the interstices of the sand, while the molten metal will occupy the void created by vaporization of the foam pattern to provide a cast metal part having a configuration conforming to the configuration of the pattern.